This invention relates generally to sealed-cavity microstructures fabricated using semiconductor fabrication processes and, more particularly, to microstructures having internal cavities sealed with both a pressure seal bond and a structural bond as well as associated fabrication methods.
A wide variety of sensors, actuators and other devices may be miniaturized using silicon integrated circuit processing technology. For example, microelectromechanical structures (MEMS) comprise electromechanical devices, such as relays, accelerometers and actuators, formed on a silicon substrate using integrated circuit fabrication processes. MEMS devices and other microstructures promise the benefits of small size, low cost, high quality, high reliability, and low power consumption.
In the fabrication of MEMS and other microstructures, it is sometimes required that two substrates be structurally integrated together, such as by structural bonding. The required structural bonds can be provided by any of several bonding techniques known in the art. For example, a direct bond may be formed by joining two clean, polished surfaces together under compressive force at an elevated temperature. Alternatively, two adjacent solder structures may be integrated by reflowing the solder at an elevated temperature. In addition, an anodic bond may be formed between an insulating substrate and a conducting or semiconducting substrate by the application of a high voltage, such as 1,000 volts, across the junction at an elevated temperature. Structural bonds such as the aforementioned are well developed for providing mechanical integration of two or more microstructures.
In some microstructure applications, a pressure seal may be desired, such as to isolate a cavity internal to a MEMS or other microstructure from the surrounding environment. Pressure seals may be required, for instance, when a high pressure gas atmosphere is desired inside a cavity, such as to increase the breakdown voltage threshold. In other applications, an evacuated cavity may be required, such as for improving the thermal isolation of suspended radiation detectors in a microbolometer. Unfortunately, common structural bonding techniques are generally inadequate to provide pressure sealing because of surface variations and imperfections that preclude the formation of a tight seal across the full extent of a structural bond. It is particularly difficult to provide a tight pressure seal if electrical signals must enter and exit the cavity, such as through electrical feedthroughs or wires.
It is known in the art to provide a pressure seal for a microstructure cavity by the deposition of a layer of a deformable metal onto a dielectric seal ring about the periphery of the cavity. For example, gold, tin, lead, or indium may be deposited, such as by sputtering, on a silicon dioxide ring formed on the periphery of two mating wafers to form a sealed-cavity microstructure assembly. When two mating wafers defining a cavity therebetween are joined under moderate compressive force, the deposited metal on the seal ring deforms to adapt to any surface variations and forms a tight seal around the periphery of the cavity formed between the wafers. Unfortunately, pressure seals formed in this manner are not generally structurally sound and devices thus formed are not rugged.
Wafer scale batch processing delivers substantial economies of scale to reduce manufacturing costs in the fabrication of microstructures. It is known in the art to fabricate portions of a plurality of microstructures on each of two wafers that are thereafter structurally bonded to form multiple microstructure devices. The wafers can then be divided into the individual devices after substantially all microstructure fabrication operations have been completed. Conventional sawing or etching operations can be used to accomplish individual device separation at the wafer level. Sealed-cavity microstructures with pressure-tight through-the-seal electrical feedthroughs, however, are not conventionally processed at the wafer level because available pressure seals cannot withstand the required processing.
Unfortunately, microstructures based on evacuated-cavity or pressurized-cavity devices have not been widely adopted in industry because of the high cost to manufacture microstructures with well-sealed cavities in large quantities. Attempts to utilize low-cost wafer-scale batch processing, have generally met with failure due to device design limitations imposed by the lack of an adequate general purpose pressure-tight seal that accommodates through-the-seal electrical feedthroughs and is compatible with wafer-scale batch processing techniques.
A microbolometer focal plane array is a form of MEMS device that uses an array of small radiation detector elements suspended above a semiconductor substrate. As radiated energy, such as infrared energy, is received by each detector element, the temperature of the detector element increases. The resulting temperature-induced change in the resistance of each of the detector elements is detected by a multiplexing integrated circuit formed on the semiconductor substrate. The small radiation detectors of a microbolometer focal plane array are fabricated using micro-machining techniques known in the MEMS art. Microbolometers are generally very sensitive radiation sensors because each of the individual radiation detectors has a very small thermal mass and is thermally isolated from the multiplexing integrated circuit and surrounding structure.
A representative microbolometer array is disclosed by U.S. Pat. No. 5,627,112 to Tennant et al. (xe2x80x9cTennantxe2x80x9d). Tennant discloses a multiplexer wafer 124 containing integrated multiplexer circuitry. A wafer 122 on which there is formed an array of radiation detectors is attached to multiplexer wafer 124. The wafer attachment is accomplished by use of a micro-positioning alignment tool which applies force and heat to form compression welds between corresponding columns of indium metal on each of the two wafers. The height of the welded indium columns determines the spacing between the two wafers and thus the separation of the suspended microstructures from the multiplexer. The space between the two wafers is filled with a temporary epoxy, and mechanical lapping and then etching are used to remove the substrate under the radiation detector structures. After the individual thermal detector arrays have been diced apart to form individual devices, a plasma is used to remove the epoxy around and between the individual radiation detector elements in each array. The resulting individual radiation detector elements are free-standing, being connected only to the multiplexer wafer 124. The radiation detectors are not enclosed within a vacuum-sealed cavity.
A vacuum-sealed Fabry-Perot detector microstructure employing a microbolometer is disclosed by U.S. Pat. No. 5,550,373 to Cole et al. (xe2x80x9cColexe2x80x9d). Cole discloses wafer 32 vacuum-sealed to bolometer detector 24 with a continuous support 30 around the perimeter of wafer 32 and the wafer 20 upon which the bolometer detector 24 is mounted. Support 30 encloses microbolometer 12 and Fabry-Perot cavity 18 in a vacuum. The Cole microstructure does not include additional bonding between wafers 20 and 32 and is vacuum-sealed at the individual device level.
Another known microstructure is provided by U.S. Pat. No. 5,701,008 to Ray et al. (xe2x80x9cRayxe2x80x9d). Ray discloses infrared detector array pixels 6 inside a vacuum-sealed Dewar microstructure assembly that includes a getter 15 to remove residual gas molecules from the sealed cavity. A seal 8, supporting an infrared window 10, can be formed with indium, tin or lead solder or by a vacuum epoxy. Neither the Cole nor the Ray microstructures include additional bonding between wafers and both are vacuum-sealed at the individual device level.
Conventional sealed-cavity microstructures, therefore, suffer from the limitations discussed above. In particular, conventional structural bonding techniques are inadequate to provide pressure sealing, while conventional pressure seals, conversely, are not structurally sound. In addition, sealed-cavity microstructures, such as conventional vacuum-sealed microbolometers, are not adapted to wafer-level batch processing because the pressure seals cannot withstand the required processing environments. Further attempts to fabricate microstructures having sealed cavity by means of low-cost wafer-scale batch processing have generally met with failure due to mutually exclusive materials and process requirements. As a result, microstructures with well-sealed cavities are expensive to produce and have not been widely adopted in industry.
It is an object of the present invention to provide a sealed-cavity microstructure, and an associated fabrication method, which incorporates both a tight pressure seal and a rugged structural bond to enable low-cost wafer-scale batch processing.
It is a further object of the present invention to provide a rugged vacuum-cavity microbolometer, and an associated fabrication method, adapted to be produced using low-cost wafer-scale batch processing.
These and other objects are provided, according to the present invention, by a method of fabricating a microstructure with at least one pressure-maintained cavity, which joins first and second wafers such that a recessed portion of at least one of the wafers forms a cavity therebetween and which thereafter forms both a pressure seal and a structural bond about the cavity. The resulting microstructure is structurally integrated as a result of the structural bond and includes an internal cavity that can be maintained at a predetermined pressure due to the surrounding pressure seal. Thus, not only is the resulting microstructure rugged, but the method of the present invention facilitates lower cost wafer-style batch processing of microstructures having pressure-maintained cavities.
In one embodiment of the invention, at least one of the wafers defines a seal ring that extends peripherally about the cavity. By depositing a deformable metal on the seal ring, the pressure seal can be formed on the seal ring as the first and second wafers are brought together. In addition, the structural bond, such as an anodic bond, a direct bond, a micro-velcro bond or a solder bond, can be formed outside of and peripherally about at least a portion of the seal ring to structurally integrate at least portions of the first and second wafers.
A microbolometer is also provided according to the invention that includes first and second wafers defining a cavity therebetween, at least one radiation detector suspended within the cavity, a vacuum seal between the two wafers that extends peripherally about the cavity to maintain a vacuum within the cavity, and a structural bond between the two wafers to structurally integrate at least portions of the two wafers.
The microstructure and associated fabrication methods of the present invention therefore overcome limitations imposed by conventional sealed-cavity microstructures. In particular, a sealed-cavity microstructure is provided that incorporates both a tight pressure seal and a rugged structural bond. A sealed-cavity microstructure and a vacuum-cavity microbolometer are thereby provided that are adapted to low-cost wafer-scale batch processing.